The field of the present invention is radial inflow turbines, also known as turboexpanders, and more specifically, variable primary nozzle systems of radial inflow turbines.
Radial turbines employ an annular inlet surrounding a turbine wheel through which is directed influent under pressure. To uniformly distribute the influent, primary, stationary vanes are disposed about the annular inlet to create a nozzle therebetween. These nozzles are often variable through the controlled pivotal motion of the primary vanes.
The primary vanes are typically mounted between mounting rings. One of the mounting rings may be pivotally mounted relative to the other mounting ring which is then employed as a means for pivoting the vanes. The mounting rings are also mounted for relative axial movement therebetween. Normally, one ring is fixed while the other is allowed to move axially to accomplish this result. Typically, a pneumatic or hydraulic cylinder is associated with the pivotal mounting ring to forcefully control the position of the mounting ring, in turn controlling the vanes. One such system is presented in U.S. Pat. No. 3,495,921 directed to Variable Nozzle Turbine, the disclosure of which is incorporated herein by reference.
Because of the inherent pressures in such radial turbines, particularly the static and dynamic pressures of the flow through the primary nozzle, clamping forces are applied by the mounting rings to the sides of the vanes adjacent the mounting rings if one or both of the rings is axially movable. A close fit of the rings about the vanes prevents the occurrence of "blow-by," i.e. direct leakage flow from inlet to outlet, bypassing the nozzle and reducing turbine efficiency. However, the resulting clamping forces often can become excessive, and actuation of the vanes to adjust the nozzle is inhibited.
A method of attempting to control clamping forces is disclosed in U.S. Pat. No. 4,502,836, directed to Method For Nozzle Clamping Force Control, the disclosure of which is incorporated herein by reference. In the referenced patent, a fluid pressure driven actuator system, driven by two supply lines, varies the pressure to an annular space adjacent a mounting ring. During steady state operation, the nozzle is clamped by the associated mounting rings. When pressure builds in the actuator system for adjusting the nozzle, the annular space is pressurized to overcome the clamping force. See also earlier U.S. Pat. Nos. 4,242,040 and 4,300,869, the disclosures of which are incorporated herein by reference.